This invention relates to separators for use in alkaline storage batteries, and particularly for use in batteries having electrode systems such as nickel-cadmium, nickel-zinc, silver-zinc, and manganese dioxide-zinc, in which the separator must be highly resistant to physical and chemical influences within the cell.
It has heretofore been common practice in the art of secondary alkaline cells to employ inter-electrode separators containing a plurality of layers, at least one of which is a membranous material such as cellophane. This membrane layer must be compatible with the alkaline electrolyte within the cell, be sufficiently permeable to allow passage of electrolyte ions, yet not be so highly permeable as to permit the passage of larger molecules or ions therethrough, e.g. electrode-derived conducting ions. Such membranous materials are particularly useful in attenuating the growth of zinc dendrites in alkaline rechargeable cells having zinc as the basis for the negative electrode. These dendrites or "trees" are produced at and propagated from the zinc electrode during charge, and eventually produce tree-like conductive branches which extend to the cathode, short out the cell and seriously reduce its life.
While cellophane and related membranous materials do retard such adverse metallic conduction between the electrodes of rechargeable alkaline cells, one extreme disadvantage is the result of the relatively fragile character of such membranes. For instance, during initial handling and separator assembly, the membranous material is easily creased, curls readily upon slight changes in humidity and may be punctured or other discontinuities or weak spots may be created. The occurence of such weak spots and discontinuities is also created during cell operation. For instance, compression of the separator often times results in projections completely severing the membrane or causes the occurrence of folds and creases. These adverse phenomena are particularly common when there is freedom of movement of the membrane layers with respect to adjacent electrodes and particularly between contiguous separator layers themselves. It is important to have a separator material uniform throughout its surface. This uniformity is particularly critical in cells having zinc anodes in which dendritic growth has been a severe problem. It has been discovered that discontinuities in the membranous material are extremely conducive to zinc growth.
The most pertinent prior art may be found in the United States Patent Office Classification Class 136, Batteries. Particularly instructive references include the following Pat. Nos. 2,534,336; 2,858,353; 2,900,433; 2,904,615; 2,930,829; 2,994,728; 3,081,372; and Canadian Pat. No. 721,815.
In one aspect of this invention it is an object to produce a novel multi-layer separator in which the use of relatively fragile membranous layers is obviated entirely.
In another aspect of this invention, it is an object to produce a multi-layer separator containing at least one membranous layer in which this membranous layer is sandwiched between and tightly adheres to contiguous bibulous layers to provide strength and protection to the membranous layer during cell assembly and operation.
It is another object to provide a separator for alkaline batteries including a plurality of layers laminated together to form an integral separator material having mechanical integrity.
It is a further object to provide a separator resistant to dendrite propagation therethrough.
These, and other objects, are met and the disadvantages of the prior art are solved by employing the separator and its method of preparation according to the present invention as described herein.